When we use our hands to explore the world around us, we receive two types of feedback -- kinesthetic and tactile. To understand the difference between the two, consider a hand that reaches for, picks up and explores a baseball. As the hand reaches for the ball and adjusts its shape to grasp, a unique set of data points describing joint angle, muscle length and tension is generated. This information is collected by a specialized group of receptors embedded in muscles, tendons and joints.

Known as proprioceptors, these receptors carry signals to the brain, where they are processed by the somatosensory region of the cerebral cortex. The muscle spindle is one type of proprioceptor that provides information about changes in muscle length. The Golgi tendon organ is another type of proprioceptor that provides information about changes in muscle tension. The brain processes this kinesthetic information to provide a sense of the baseball's gross size and shape, as well as its position relative to the hand, arm and body.

When the fingers touch the ball, contact is made between the finger pads and the ball surface. Each finger pad is a complex sensory structure containing receptors both in the skin and in the underlying tissue. There are many types of these receptors, one for each type of stimulus: light touch, heavy touch, pressure, vibration and pain. The data coming collectively from these receptors helps the brain understand subtle tactile details about the ball. As the fingers explore, they sense the smoother texture of the leather, the raised coarseness of the laces and the hardness of the ball as force is applied. Even the thermal properties of the ball are sensed through tactile receptors.

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Force feedback is a term often used to describe tactile and/or kinesthetic feedback. As our baseball example illustrates, force feedback is vastly complex. Yet, if a person is to feel a virtual object with any fidelity, force feedback is exactly the kind of information the person must receive. Computer scientists began working on devices -- haptic interface devices -- that would allow users to feel virtual objects via force feedback. Early attempts were not successful. But as we'll see in the next section, a new generation of haptic interface devices is delivering an unsurpassed level of performance, fidelity and ease of use.